How can I collect a debt

You can start with a demand letter from an attorney. You might see some progress before taking more costly steps.

If this was an oral agreement, you only have 2 years from the date of breach to sue in CA before the statute of limitations bars your claim, so seek litigation counsel NOW to preserve your rights you may already be too late.

if this was a written obligation, then you have 4 years from the date of breach to sue.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.

I agree with both previous answers and would add that information about the debtor is the key to effective collections. Where they get and where they keep there money will help your attorney intecept the flow of cash and divert it to you, and your attorney.

Good Luck